Hi to all and thanks for your comments. I'll try to answer each below:
To Hewak:
QUOTE(hewak @ Jul 22 2007, 01:58 AM)

To get a clearer message, you'll have to tune the radio into a channel with very little distrubance from other broadcasts, so find a channel that is farthest away from any of your local radio stations, probably at the beginning or end of the dial, I believe AM will give you a stronger signal.
After my grandfather's passing the radio was completely detuned. It doesn't work anymore in a "normal" way (i.e. you can't tune any radio station now, you only hear white noise whithin all the spectrum).
To JustNormal:
QUOTE(JustNormal @ Jul 22 2007, 04:22 AM)

The more you listen the more HE thinks you need him, so give him your approval that he does not have to stick around.
Thanks for your kind words... The other day he said very clearly "Listen!" and then his voice vanished. I wonder if he have an important message to deliver. Maybe regarding the mysterious circumstances of his death, two weeks ago. I don't know...
To Lonecat:
QUOTE(Lonecat @ Jul 22 2007, 06:57 AM)

Is the radio in question plugged into the mains? Or do these messages come through without power? While trying to respond to your grandfather you should ask him if he can see you or what he thinks the best means of contacting him might be. Good luck to you and to him!

The radio is battery operated and, yes, it was turned on the three times I listened to his voice. In our first communication my grandfather answer directly to my call, after I felt his presence in the room. So I'm almost sure that there is a two way communication going on. You can read the details here:
http://www.goldsteincollection.com/archives/3To Barek Halfhand:
QUOTE(Barek Halfhand @ Jul 22 2007, 01:40 PM)

Is it a 'solid state' radio?......B
Yes, it's a solid state. I took a picture of the radio. It's a Spica. Check it out:
http://www.goldsteincollection.com/archives/5Best regards to all and thanks again for your help.
Matt